This invention relates to a pressure setting device which is designed so that a positive or negative desired set pressure is applied to a controlled system by the employment of electromagnetic valves.
For example, in a variety of measurements, there are some occasions when measurement is made to observe how various physical quantities of a controlled system, such as temperature, stress, etc., vary with a positive or negative set pressure applied to the controlled system. In some cases, reference pressures of various values are applied to a variety of pressure instruments for the calibration thereof. For generating a desired set pressure for such a purpose, there has been proposed a pressure setting device of the type in which a positive or negative air pressure source and a controlled system are interconnected through a first electromagnetic valve, the controlled system is opened to the air through a second electromagnetic valve, the pressure of the controlled system is detected as an electric signal by a pressure sensor, and the first and second electromagnetic valves are selectively controlled in accordance with a deviation between the detected output and the set value.
In this kind of pressure setting device heretofore employed, a dead zone is set up about the set value and when the pressure of the controlled system is smaller in absolute value than the dead zone, the first electromagnetic value is opened and the second electromagnetic valve is held closed. Conversely, when the pressure of the controlled system is larger in absolute value than the dead zone, the first electromagnetic valve is closed and the second electromagnetic valve is held open, and when the pressure of the controlled system is in the dead zone, both electromagnetic valves are maintained in their closed state.
In the abovesaid conventional device, if electromagnetic valves of large flow rate are used, when the pressure of the controlled system enters into the dead zone, it is difficult to rapidly close both electromagnetic valves, and consequently the pressure of the controlled system readily passes through the dead zone and an operation is needed to return the pressure to the dead zone; namely so-called hunting results. This hunting can be avoided by the use of electromagnetic valves of small flow rate or by setting up a large dead zone. However, in the former case, it takes much time for the pressure of the controlled system to reach the set value, and the system accordingly exhibits poor response; and in the latter case, the accuracy of the set pressure is low.
To solve such problems, there has been proposed a pressure setting device in which there are provided in parallel a first electromagnetic valve of large flow rate for applying air pressure to the controlled system and a second electromagnetic valve of small flow rate for reducing the air pressure. With this conventional pressure setting device, when the pressure of the controlled system greatly differs from the set pressure, the electromagnetic valve of large flow rate is controlled, and when the difference between the two pressures becomes small, the electromagentic valve of small flow rate is controlled. In this case, however, four electromagnetic valves are required and their control is complicated, which results in the device becoming bulky and expensive.
If control valves whose opening can be controlled are used in place of the electromagnetic valves, response is relatively good, and the dead zone can be made small. But the control valves are expensive.
An object of this invention is to provide a pressure setting device which employs electromagnetic valves, achieves excellent response and can be produced at low cost.
Another object of this invention is to provide a pressure setting device which enables proportional control by the use of electromagnetic valves.
Still another object of this invention is to provide a pressure setting device which operates stably in the vicinity of a set pressure and causes the pressure of a controlled system to reach a set value relatively rapidly.